Thiophenes and thiazoles

T system, the PPP a—-tt method (133). The -tt and a net charges and bond orders of thiophene and thiazole are compared in Table 1-5. Whatever the method considered the variation of the indices occurs in the same sense when passing from thiophene to thiazole the replacement in the 3-position of a carbon atom by a nitrogen induces... 

A study of three commercially available extracts of yeast (Saccharomyces cerevisiae) identified 268 volatile components of which 67 contained sulfur (34 of these were identified for the first time as yeast extract components). The identified substances ranged from CS2 to complex thiophenes and thiazoles.28... 

The heterocycles discussed thus far have generally been part of the complex acceptor, with the exception of the thiophene and thiazole couplers quoted, which are not of commercial interest. 

The spin-coupled method has now been applied to a large number of aromatic systems benzene and naphthalene azobenzenes, such as pyridine, pyridazine, pyrimidine and pyrazine five-membered rings, such as furan, pyrrole, thiophen, and thiazole and inorganic heterocycles, such as borazine ( inorganic benzene ) and boroxine, for which we find little evidence of aromaticity. Structural formulae are collected in Fig. 1. For all of these molecules we have included the effects of electron correlation for the Jt electrons but not for the a framework. This a-n separation is an approximation whose utility rests upon the chemistry of aromatic systems — to abandon it would be to ignore this entire body of experience. Furthermore, very extensive calculations [4] have demonstrated that rc-electron only correlation affords an excellent description of ground and excited states of benzene. 

The spin-coupled description of the rt-electron systems in the heteroaromatic molecules furan, pyrrole, thiophen and thiazole is also based on localized but non-orthogonal orbitals. The spin-coupled orbitals for pyrrole are shown in Fig. 5. There is one orbital localized on each carbon atom and one on the heteroatom ([Pg.50]

The Maillard reaction has received much attention since the 1950 s as the source of flavor chemicals in cooked foods. Numerous compounds produced by this reaction have been reported in the last two decades. The major flavor chemicals are nitrogen- and sulfur-containing heterocyclic compounds. For example, nitrogen-containing pyrazines contribute a characteristic roasted or toasted flavor to cooked foods. Sulfur-containing thiophenes and thiazoles give a characteristic cooked meat flavor. A striking property of these compounds is their extremely low odor thresholds. 

Figure 10 Predominant thiophenes and thiazoles identified in roasted coffee (units in ppm)...

Comparison of the reactivity of thiazole, isothiazole, and thiophene (see also Chapter 6, Section 9.A) determined under the same gas-phase conditions permits determination of the effect of the aza substituent at each position in thiazole (assuming that sulfur has the same effect in both thiophene and thiazole) and likewise at the 5-position of isothiazole. This effect (in terms of cr+ values) is shown in Scheme 7.14, together with, for comparison, the values obtained for pyridine under the same conditions (see also Chapter 9). 

The reaction of sulfur-containing heterocyclic compounds such as thiophenes and thiazoles (Eq. 68) is specifically enhanced by addition of Cul, while the role is not yet definitive [135]. The use of a bulky phosphine ligand such as P(f-Bu)3 also promotes the arylation [141]. 2-Iodothiophenes are polymerized to produce polythiophenes (Eq. 69) [142]. 

Other S-containing compounds are present in Sections 5.1 (furans) and, of course, 5.J and 5.M (thiophenes and thiazoles). 

Some of the compounds identified in YEs which are formed either by the thermal degradation of thiamine or on the interaction of thiamine degradation products with other components are shown in Fig. 2. They include aliphatic sulfur compounds, furans, thiophenes and thiazoles. 2-Methyl-3-furanthiol and 2-methyl-3-thiophenethiol have been identified in YEs 9,13 14) and are well known thermal degradation products of thiamine (29). As well as possessing meaty aromas and low odor threshold values 34), these compounds are key precursors of several other sulfur-substituted furans and thiophenes, including the derivatives in Fig. 2. Most possess meaty aromas at low concentrations and several have been identified in YEs (see Tables I and III). 

In 2011, Itami and Studer [183] developed a palladium-catalyzed C4-selective arylation of thiophenes and thiazoles with arylboronic acids. Although they had already reported the C4(/J)-selective arylation of thiophenes with aryl iodides [88] (Scheme 17.18), this C-H/C-B coupling method [using a Pd"/hipy or phen/TEMPO ((2,2,6,6-tetramethylpiperidin-l-yl)oxyl) catalyst system] enabled the use of thiazoles as aryl nucleophiles. They also applied this coupling reaction to the synthesis of the core structure of SCH-785532, which is known as a BACE inhibitor. In the same year, Itami [184] reported a direct arylation of a PAH with arylboronic acids to generate a 7t-expanded PAH. Treatment of pyrene 142 with arylboroxine 143 in the presence of Pd(OAc)2 and o-chloranil as an oxidant, followed by cydiza-tion under stoichiometric FeClj, produced PAH 144. Key to the unprecedented C-H arylation was a notable combination of Pd" and o-chloranil. 

A series of 5 -thienyl derivatives were prepared using metalation chemistry. Thiazole 6 (Equation 4) was treated with 1.0 equivalent of n-butyllithium in THF at -78 C for 2 h and the resulting anion quenched with 1.1 equivalents of an electrophile. Extractive workup and chromatography afforded the 5 -substituted derivatives in 40-80% yield. In the course of these experiments, we discovered that it was necessary to avoid excess alkyl lithium in this reaction sequence (Equation 5). Under these reaction conditions, excess base metalated both the thiophene and thiazole rings, resulting in a mixture of mono- and bis-alkylated thienyl-thiazoles. 

Covalent phosphorus compounds (sugar phosphates, phytic acid and nucleotides) have already been mentioned several times. Foods also contain many organic sulfur compounds, such as sulfur-containing amino acids, thiols, (oHgo)sulfides, glucosi-nolates, heterocyclic compounds (derivatives of thiophene and thiazole). Their occurrence, properties, reactions and importance are covered elsewhere in this book. 

Lee J, Chung JW, Jang J, Kim DH, Park JI, Lee E, Lee BL, Kim JY, Jung JY, Park JS, Koo B, Jin YW, Kim DH (2013) Influence of aikyi side chain on the crystallinity and trap density of states in thiophene and thiazole semiconducting copolymer based inkjet-printed field-effect transistors. Chem Mater 25 1927-1934... 

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